Most waters contain ions and other charged species such as colloids. When such waters flow over or through flow channels, a zeta potential forms on the surfaces of the flow channels. When the flow channel surface, in the presence of charged species in water, is moved past a pair of electrodes, a current is generated. This current called the streaming current, and it is proportional to the net charge density of the water flow.
Measuring the streaming current is useful in controlling the amount of chemicals needed to add to fluids for various chemical processes, such as treating water for consumption or disposal. The theory of operation of such streaming current sensors is discussed extensively in U.S. Pat. Nos. 3,368,145 and 4,961,147, incorporated herein by reference. As discussed in those and other patents, a primary use of streaming current sensors is to determine whether chemically treated water is safe for consumption. For example, to make water clean enough for use, treatment chemicals are employed to change the charge density of the water so that contaminants in the water will form aggregates and settle out of the water as a floc. In particular, raw water that is usually negatively charged is processed with coagulant chemicals such as alum to reduce the negative charge. In most, if not all cases, it is economically desirable to minimize the use of the chemicals for floc formation.
Once water has been chemically treated, any floc which forms has a tendency to stick to any surface it comes into contact with, especially to horizontal surfaces. If floc is allowed to build up on a surface disposed proximate a probe in which the streaming current is being measured to evaluate the charge density of a flow stream, the sample will give an unreliable reading of the streaming current. Most successful streaming current sensors use a piston-electrode chamber configuration. Attempts have been made to keep the probe and its electrodes clean during operation. Canzoneri et al., U.S. Pat. No. 4,446,435, uses an ultrasonic cleaner attached to a probe to clean the area around the probe during operation. Ultrasonic cleaning does tend to remove colloidal particles from the surfaces of the piston-electrode chamber where measurements upon a sample are made; however, this method of introducing the sample flow into the bottom of the housing of the probe and discharging the sample flow from the housing near the top of the probe works against the natural flow of the floc with the result that the sample flow tends to be self-contaminating. Canzoneri attempted to rectify this result in the invention disclosed U.S. Pat. No. 4,449,101. In this improvement, a periodic wash is included which a cleaning fluid was backflowed into the piston-electrode chamber. Although this backflow also helped to alleviate the contamination problem, its inclusion also made the indicator system considerably more complicated.
Gerdes, U.S. Pat. No. 3,368,145, discloses a device in which the sample stream is introduced at the top of the probe and discharged downwardly from the side of the probe. However, Gerdes allows the sample stream to enter a reservoir about the piston electrode chamber within the probe so that some settling of floc and other particles tends to occur before the sample flows into the piston-electrode chamber. This causes difficulty in use of the apparatus under field conditions, resulting in erratic and erroneous measurements after only a few hours field service.
Moore, in U.S. Pat. Nos. 4,297,640 and 4,961,147, discloses units which attempt to reduce noise which is generated in the signal due to a buildup of floc near the top of the upper electrode. To minimize these noise effects, Moore places a grounding guard electrode above the two sensing electrodes, between the sensing electrodes and the point of floc buildup. This approach reduces the current that could have been produced in the absence of a grounding electrode, resulting in a weaker signal. Further, the guard electrode, while effective, does not completely eliminate all potential galvanic interferences.
Bryant, U.S. Pat. No. 4,769,608 discloses a vertical tubular flowpath in which the test flow stream constantly washes a transverse passageway to prevent contaminant accumulation. The sample is sucked into capillary sized channels within a piston-electrode chamber. The sample flows over the entrances to the capillary-sized channels with sufficient force to remove any floc that may accumulate. Bryant et al., U.S. Pat. No. 5,119,029, improves upon the invention of U.S. Pat. No. 4,769,608 by providing a streaming current sensor in which the holder for the electrodes can be removed and replaced quickly and easily.
Despite the numerous improvements made by the prior art, only limited improvement in long-term stability and sensitivity has been made over the earlier inventions. Unfortunately, the long term stability and sensitivity of currently known sensors are not as good as is sought after in industry. The sensors of the prior art locate the upper end of the flow path member exposed or vicinal to the flow liquid stream passageway (U.S. Pat. Nos. 4,961,147 and 5,119,029). It is believed in the art that this structure will keep the electrodes and the dielectric surface proximate to those electrodes free from floc buildup which would otherwise decrease the sensor's sensitivity. While partially correct, this belief fails to recognize that the sample liquid passing through the passageway is not homogeneous. This causes the sensor signal to fluctuate (under stable conditions, the second decimal digit may fluctuate in currently known sensors) and contributes to the poor stability of the sensors.
Further, sensors known in the art suffer from reduced sensitivity. The instrument's sensitivity deteriorates as the instrument operates over a period of time. The main cause behind the problem is that there is no reasonable passageway available to allow contaminants (floc for example) to exit the flowpath member before they stick to sensing electrodes and the dielectric surface proximate to these electrodes.
Most improvements in this field have been directed to self cleaning (U.S. Pat. No. 5,119,029) or employment of a third grounded electrode to collect contaminants (U.S. Pat Nos. 4,297,640 and 4,961,147). However, the sensors of the prior art mainly use passive means to handle contaminants. That is, they first allow contaminants stick to the instrument's sensitive parts, then they take some action to remove them. It would, therefore be highly advantageous to provide a sensor in which the accumulation of contaminants is prevented, thereby maintaining a high level of stability and sensitivity over a long period of time.